Well it sort of does matter. People were excited about Zaralek at first, and clearly the gameplay sidelined that excitement. That shows that people are willing to give an Underground experience a shot, Blizzard just has to provide the proper gameplay for it.
Well the simple reality is that both the underground sections of Azeroth and Goblins and Gnomes are integral parts of Warcraft. There are fantasy fans who like underground areas and concepts, there are Warcraft fans who like Goblins and Gnomes, and there are fantasy and warcraft fans who like steampunk fantasy. Every expansion isn't going to appeal to every player, and it isn't good for the long term health of the game to not take risks. For example, I personally didn't want or like Shadowlands, but I appreciated Blizzard doing the expansion and taking the risk. Same applies for MoP. I'm sure plenty of standard WC players felt MoP was the dumbest concept ever, but in the end WoW is better for it happening instead of Blizzard playing it safe.
I believe an underground expansion is in that same vein of risk taking, which is why I believe it's highly likely to happen.
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Gee, I have no idea....